Materials which have high coercive forces (Hc), and smooth surfaces have been regarded as good media for high density recording. As recording media for this purpose, recording media based on magnetic powders and recording media based on thin metal films formed by the evaporation, metal plating or other processes have been used. However, both types of recording media suffer from a lack of stability because they are liable to be oxidized, and attempts to put them to practical use have been unsuccessful.
In recent years, to attain high output levels in short-wave length recording systems, recording systems which utilize (1) the vertical component of the magnetic field used for recording or (2) perpendicular magnetic recording have been proposed.
Although these systems are excellent in principle, excellent recording media useful therein are not available.
Specifically, using a magnetic material made of conventional needle-shaped magnetic particles oriented at random has been proposed. However, such a material does not provide sufficient output due to the low relative density of the vertical component oriented in the perpendicular direction.
Further, the use of needle-shaped magnetic particles oriented perpendicular to the plane of the magnetic layer has been proposed, and a number of methods for orienting such magnetic particles have also been proposed. However, sufficient orientation has not been achieved with these methods as the initially vertically oriented needle-shaped particles tilt down toward a plane parallel to the surface of the magnetic layer upon contraction of the magnetic coating in its thickness direction on drying.
In addition, recording media comprising a thin metal film formed by sputtering or evaporation techniques for perpendicular magnetic recording have been proposed. However, such are insufficient in terms of chemical stability, magnetic head abrasion resistance and durability.
In the present invention, the term "perpendicular magnetic recording" refers to the recording process in which a magnetic head as illustrated in FIG. 1 is used; such a process is described in Television Gakkaishi, Vol. 32, No. 5, p. 399 (1978) or IEEE Transactions on Magnetics 1980 (1) Vol. MAG-16, November 1, p 71.
On the other hand, the expression "the recording system utilizing the vertical component of magnetization" refers to the recording system in which the vertical component of a magnetic field created by a ring head on the surface of a magnetic layer, as illustrated in FIG. 2, is utilized; such a system is described in IEEE Transactions on Magnetics Vol. 15, No. 6, November 1979. When a head having a narrow gap is used, recording is, in general, carried out very efficiently.
It is known that using a magnetic material of high coercive force to raise the output in short wave length recording results in lowering output in long wave length recording. To solve this problem, the use of a magnetic layer comprising two layers, a lower layer having low coercivity and a high magnetic flux density and an upper layer having high coercivity force has been proposed. As the upper layer, a metallic magnetic powder such as cobalt modified iron oxide powder, CrO.sub.2 powder or the like, a metal thin film formed by an evaporation process, a metal plating process or the like, and the like are known. Each of these techniques exhibits good characteristics within certain limits. However, there are limits on the characteristics attainable using these techniques as magnetic material present in the vicinity of the surface of a magnetic layer has the tendency to orient in a plane parallel to the surface and, therefore, the vertical magnetization component in the vicinity of the surface contributes little to recording.
Processes for orienting magnetic particles (contained in an upper layer) perpendicular to the layer plane have also been proposed. However, since the magnetic materials employed in such processes were made of needle-shaped magnetic particles, sufficient orientation in the perpendicular direction was not achieved due to the "tilting" effect mentioned, and, consequently, satisfactory characteristics were not obtained.